Anti-Solvent Technique for Recovering an Organic Solvent from a Polyarylene Sulfide Waste Sludge

ABSTRACT

Methods and systems are provided for recovering an organic solvent from a waste sludge generated during formation of a polyarylene sulfide. Methods include combining the waste sludge with an anti-solvent to create a dispersion, which includes a solid phase that includes a substantial portion of the impurities of the polyarylene sulfide formation and a liquid phase that contains the anti-solvent and organic solvent employed during formation of the polyarylene sulfide. The liquid phase is separated from the solid phase and then subjected to a distillation process to separate the organic solvent from the anti-solvent. Methods can also include forming the polyarylene sulfide by a polymerization process and thereafter purifying a slurry of the polyarylene sulfide. A liquid washing product is formed as a result of the purification process, which can be subjected to a distillation process that forms an organic solvent-rich stream and the waste sludge.

RELATED APPLICATION

The present application is based upon and claims priority to U.S.Provisional Patent Application Ser. No. 63/241,665, having a filing dateof Sep. 8, 2021, which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

Polyarylene sulfides are generally formed via polymerization of adihaloaromatic monomer with an alkali metal sulfide or an alkali metalhydrosulfide in an organic amide solvent. Following formation, thepolyarylene sulfide is washed with an organic solvent (e.g.,N-methylpyrrolidone) to purify the polymer. During this process, one ormore solvent-containing streams are generated and thereafter subjectedto a series of distillation steps to recover the organic solvent. Awaste stream (“sludge”) is also formed during the distillation processthat must be disposed off-site. Unfortunately, this waste sludge stillcontains a fairly significant amount of the organic solvent. As such, aneed currently exists for an improved method and system for effectivelyrecovering an organic solvent from the waste sludge generated duringformation of a polyarylene sulfide.

SUMMARY OF THE INVENTION

In accordance with one embodiment of the present invention, a method isdisclosed for recovering an organic solvent from a process used to forma polyarylene sulfide. The method can include contacting a waste sludgedeveloped in formation of a polyarylene sulfide with an anti-solvent toform a dispersion that includes a solid phase and a liquid phase. Thewaste sludge can contain from about 10 wt. % to about 90 wt. % of anorganic solvent. A method can also include separating the solid phasefrom the liquid phase, wherein the liquid phase contains the organicsolvent and the anti-solvent; and subjecting the liquid phase to adistillation process during which the organic solvent is separated fromthe anti-solvent and recovered. In some embodiments, a method can alsoinclude forming the polyarylene sulfide, for instance according to aprocess that includes forming a polyarylene sulfide slurry; washing thepolyarylene slurry to form a liquid washing product; and subjecting theliquid washing product to a second distillation process that generatesthe waste sludge. In some embodiments, the waste sludge can be acidifiedprior to or in conjunction with contacting the waste sludge with theanti-solvent. For instance, the waste sludge can be acidified to a pH ofabout 7 or less.

In accordance with another embodiment of the present invention, a systemis disclosed that comprises an anti-solvent stream that is configured tocontact a waste sludge developed in formation of a polyarylene sulfideto form a dispersion that includes a solid phase and a liquid phase; asolid/liquid separation system that is configured to separate thedispersion into a solid phase and a liquid phase containing the organicsolvent and the anti-solvent; and a distillation system that isconfigured to recover the organic solvent from the separated liquidphase. In some embodiments, a system can also include a polymerizationsystem that is configured to generate the polyarylene sulfide; apurification system that is configured to receive a polyarylene sulfideslurry containing the polyarylene sulfide and generate a liquid washingproduct; and a second distillation system that is configured to receivethe liquid washing product and generate a waste sludge.

BRIEF DESCRIPTION OF THE FICIURES

The present disclosure may be better understood with reference to thefollowing figures:

FIG. 1 is a schematic diagram illustrating one embodiment of a processand system of the present invention;

FIG. 2 illustrates one embodiment of a sedimentation column that may beemployed in the present invention;

FIG. 3 illustrates a cross-sectional top view of the middle section ofthe sedimentation column of FIG. 2 at the slurry inlet;

FIG. 4 illustrates several different embodiments of longitudinalcross-sectional shapes of a middle section of a sedimentation columnthat may be employed in the present invention;

FIG. 5 illustrates one embodiment of a washing system that may beemployed in the present invention;

FIG. 6 presents the organic solvent content of a filtrate following eachof several re-slurry/filter steps of an organic solvent recoveryprocess;

FIG. 7 compares the filterability of a waste sludge at pH 7 and at pH 4;and

FIG. 8 illustrates the agglomeration of solids in a slurry at variousacidification levels.

DETAILED DESCRIPTION

It is to be understood by one of ordinary skill in the art that thepresent discussion is a description of exemplary embodiments only, andis not intended as limiting the broader aspects of the presentinvention.

Generally speaking, the present invention is directed to methods andsystems for recovering an organic solvent from a waste sludge that isgenerated during formation of a polyarylene sulfide. More particularly,a polyarylene sulfide slurry is initially formed by a polymerizationprocess and thereafter purified by a washing process. A liquid washingproduct is thus formed, which is subjected to a first distillationprocess to form an organic solvent-rich stream and a separate wastesludge.

Referring to FIG. 1 , a method and system is illustrated. As shown, asystem can include a polymerization system 500 that results in formationin a polyarylene sulfide slurry 501. Following formation, thepolyarylene sulfide slurry can be passed to a purification system 600,within which the polyarylene sulfide slurry is contacted with a washingsolution (not shown in FIG. 1 ), which results in a purified polyarylenesulfide stream 601 and a liquid washing product 603. The liquid washingproduct 603 can then be further treated, for instance via a distillationsystem 700 to generate an organic solvent-rich stream 701 and a wastesludge 703. In some embodiments, the waste sludge 703 can be combinedwith a stream 706 that includes an acidification agent, and theresulting acidified waste sludge 704 can then be combined 710 with ananti-solvent 705 to create a dispersion 707, which includes a solidphase and a liquid phase. The solid phase is formed by precipitation ofa substantial portion, if not all, of the impurities in the wastesludge, such as arylene sulfide oligomers, cyclic polyarylene sulfides,fine polyarylene sulfide particles, and sodium chloride. The liquidphase, on the other hand, contains the anti-solvent and organic solvent(e.g., N-methylpyrrolidone) employed during formation of the polyarylenesulfide. One benefit of this dispersion is that the liquid phase 803 canbe readily separated from the solid phase 801 using known solid/liquidseparation techniques 800, such as filtration, centrifugation, etc. Onceseparated, the liquid phase 803 is then subjected to a seconddistillation process 900 to separate and recover the organic solvent 901from recoverable anti-solvent 903. The recovered organic solvent maythen be used again, such as in the polymerization system duringsynthesis of the polyarylene sulfide, in the purification system, etc.,as can the recovered anti-solvent, such as in the organic solventrecovery system.

I. Polymerization System

As illustrated, the system shown in FIG. 1 includes a polymerizationsystem 500 that results in the formation of a polyarylene sulfide slurry501. The polyarylene sulfide in the slurry 501 generally has repeatingunits of the formula:

-[(Ar¹)_(n)-X]_(m)-[(Ar²)_(i)-Y]_(j)-[(Ar³)_(k)-Z]_(l)-[(Ar⁴)_(o)-W]_(p)-

wherein,

Ar¹, Ar², Ar³, and Ar⁴ are independently arylene units of 6 to 18 carbonatoms;

W, X, Y, and Z are independently bivalent linking groups selected from—SO₂—, —S—, —SO—, —CO—, —O—, —C(O)O— or alkylene or alkylidene groups of1 to 6 carbon atoms, wherein at least one of the linking groups is —S—;and

n, m, i, j, k, l, o, and p are independently 0, 1, 2, 3, or 4, subjectto the proviso that their sum total is not less than 2.

The arylene units Ar¹, Ar², Ar³, and Ar⁴ may be selectively substitutedor unsubstituted. Advantageous arylene units are phenylene, biphenylene,naphthylene, anthracene and phenanthrene. The polyarylene sulfidetypically includes more than about 30 mol %, more than about 50 mol %,or more than about 70 mol % arylene sulfide (—S—) units. For example,the polyarylene sulfide may include at least 85 mol % sulfide linkagesattached directly to two aromatic rings. In one particular embodiment,the polyarylene sulfide is a polyphenylene sulfide, defined herein ascontaining the phenylene sulfide structure —(C₆H₄—S)_(n)— (wherein n isan integer of 1 or more) as a component thereof.

The polyarylene sulfide may be a homopolymer or copolymer. For instance,selective combination of dihaloaromatic compounds may result in apolyarylene sulfide copolymer containing not less than two differentunits. For instance, when p-dichlorobenzene is used in combination withm-dichlorobenzene or 4,4′-dichlorodiphenylsulfone, a polyarylene sulfidecopolymer may be formed containing segments having the structure offormula:

and segments having the structure of formula:

or segments having the structure of formula:

The polyarylene sulfide(s) may be linear, semi-linear, branched orcrosslinked. Linear polyarylene sulfides typically contain 80 mol % ormore of the repeating unit —(Ar—S)—. Such linear polymers may alsoinclude a small amount of a branching unit or a cross-linking unit, butthe amount of branching or cross-linking units is typically less thanabout 1 mol % of the total monomer units of the polyarylene sulfide. Alinear polyarylene sulfide polymer may be a random copolymer or a blockcopolymer containing the above-mentioned repeating unit. Semi-linearpolyarylene sulfides may likewise have a cross-linking structure or abranched structure introduced into the polymer a small amount of one ormore monomers having three or more reactive functional groups.

Various techniques may generally be employed in the polymerizationsystem 500. By way of example, a process for producing a polyarylenesulfide may include reacting a material that provides a hydrosulfide ion(e.g., an alkali metal sulfide) with a dihaloaromatic compound in anorganic amide solvent. The alkali metal sulfide may be, for example,lithium sulfide, sodium sulfide, potassium sulfide, rubidium sulfide,cesium sulfide or a mixture thereof. When the alkali metal sulfide is ahydrate or an aqueous mixture, the alkali metal sulfide may be processedaccording to a dehydrating operation in advance of the polymerizationreaction. An alkali metal sulfide may also be generated in situ. Inaddition, a small amount of an alkali metal hydroxide may be included inthe reaction to remove or react impurities (e.g., to change suchimpurities to harmless materials) such as an alkali metal polysulfide oran alkali metal thiosulfate, which may be present in a very small amountwith the alkali metal sulfide.

The dihaloaromatic compound may be, without limitation, ano-dihalobenzene, m-dihalobenzene, p-dihalobenzene, dihalotoluene,dihalonaphthalene, methoxy-dihalobenzene, dihalobiphenyl, dihalobenzoicacid, dihalodiphenyl ether, dihalodiphenyl sulfone, dihalodiphenylsulfoxide or dihalodiphenyl ketone. Dihaloaromatic compounds may be usedeither singly or in any combination thereof. Specific exemplarydihaloaromatic compounds may include, without limitation,p-dichlorobenzene; m-dichlorobenzene; o-dichlorobenzene;2,5-dichlorotoluene; 1,4-dibromobenzene; 1,4-dichloronaphthalene;1-methoxy-2,5-dichlorobenzene; 4,4′-dichlorobiphenyl;3,5-dichlorobenzoic acid; 4,4′-dichlorodiphenyl ether;4,4′-dichlorodiphenylsulfone; 4,4′-dichlorodiphenylsulfoxide; and4,4′-dichlorodiphenyl ketone. The halogen atom may be fluorine,chlorine, bromine or iodine, and two halogen atoms in the samedihalo-aromatic compound may be the same or different from each other.In one embodiment, o-dichlorobenzene, m-dichlorobenzene,p-dichlorobenzene or a mixture of two or more compounds thereof is usedas the dihalo-aromatic compound. As is known in the art, it is alsopossible to use a monohalo compound (not necessarily an aromaticcompound) in combination with the dihaloaromatic compound in order toform end groups of the polyarylene sulfide or to regulate thepolymerization reaction and/or the molecular weight of the polyarylenesulfide.

Although by no means required, the polyarylene sulfide can, in certainembodiments be formed in a multi-stage process that includes at leasttwo separate formation stages. One stage of the formation process mayinclude reaction of a complex that includes a hydrolysis product of anorganic amide solvent and alkali metal hydrogen sulfide with adihaloaromatic monomer to form a prepolymer. Another stage of theprocess may include further polymerization of the prepolymer to form thefinal product. Optionally, the process may include yet another stage inwhich the organic amide solvent and an alkali metal sulfide are reactedto form the complex. If desired, the different stages may take place indifferent reactors. The utilization of separate reactors for each of thestages may decrease cycle time, as the total cycle time may be equal tothat of the slowest stage, rather than the sum of all stages as in asingle reactor system. In addition, the utilization of separate reactorsmay decrease capital costs, as smaller reactors may be utilized thanwould be necessary for the same size batch in a single reactor system.Moreover, as each reactor need only meet the specifications of the stagebeing carried out in that reactor, a single, large reactor that meetsthe most stringent parameters of all stages of the formation process isno longer necessary, which may further decrease capital costs.

In one embodiment, for instance, the polymerization system 500 mayemploy a multi-stage process may be employed that utilizes at least twoseparate reactors. The first reactor may be utilized for a first stageof the process during which an organic amide solvent and an alkali metalsulfide react to form a complex that includes a hydrolysis product ofthe organic amide solvent (e.g., an alkali metal organic aminecarboxylic acid salt) and an alkali metal hydrosulfide. Exemplaryorganic amide solvents as may be used in a forming the polyarylenesulfide may include, without limitation, N-methyl-2-pyrrolidone;N-ethyl-2-pyrrolidone; N,N-dimethylformamide; N,N-dimethylacetamide;N-methylcaprolactam; tetramethylurea; dimethylimidazolidinone;hexamethyl phosphoric acid triamide and mixtures thereof. The alkalimetal sulfide may be, for example, lithium sulfide, sodium sulfide,potassium sulfide, rubidium sulfide, cesium sulfide or a mixturethereof. An alkali metal sulfide may also be generated in situ. Forinstance, a sodium sulfide hydrate may be prepared within the firstreactor from sodium hydrogen sulfide and sodium hydroxide that may befed to the reactor. When a combination of alkali metal hydrogen sulfideand alkali metal hydroxide are fed to the reactor to form the alkalimetal sulfide, the molar ratio of alkali metal hydroxide to alkali metalhydrogen sulfide may be between about 0.80 and about 1.50. In addition,a small amount of an alkali metal hydroxide may be included in the firstreactor to remove or react impurities (e.g., to change such impuritiesto harmless materials) such as an alkali metal polysulfide or an alkalimetal thiosulfate, which may be present in a very small amount with thealkali metal sulfide.

The feed to the first reactor may include sodium sulfide (Na₂S) (whichmay be in the hydrate form), N-methyl-2-pyrrolidone (NMP) and water.Reaction between the water, sodium sulfide and the NMP may form acomplex including sodium methylaminobutyrate (SMAB—a hydrolysis productof NMP) and sodium hydrogen sulfide (NaSH) (SMAB-NaSH) according to thefollowing reaction scheme:

According to one embodiment, a stoichiometric excess of the alkali metalsulfide may be utilized in the first stage reactor, though this is not arequirement of the formation stage. For instance, the molar ratio oforganic amide solvent to sulfur in the feed may be from 2 to about 10,or from about 3 to about 5, and the molar ratio of water to the sulfursource in the feed may be from about 0.5 to about 4, or from about 1.5to about 3.

During the formation of the complex, the pressure within the firstreactor may be held at or near atmospheric pressure. To maintain the lowpressure reaction conditions, vapor may be removed from the reactor. Themain constituents of the vapor may include water and hydrogen sulfideby-product. Hydrogen sulfide of the vapor can, for instance, beseparated at a condenser. A portion of the water that is separated atsuch a condenser may be returned to the reactor to maintain the reactionconditions. Another portion of the water may be removed from the processso as to dehydrate the SMAB-NaSH solution formed in the first stage. Forinstance, the molar ratio of water to NaSH (or the ratio of oxygen tosulfur) in the product solution of the first reactor may less than about1.5, or may be between about 0.1 and about 1 such that the SMAB-NaSHcomplex solution that is fed to the second stage reactor isnear-anhydrous.

Once formed, the SMAB-NaSH complex may then be fed to a second reactorin conjunction with a dihaloaromatic monomer (e.g., p-dichlorobenzene)and a suitable solvent so as to form the polyarylene sulfide prepolymerin the second stage of the process. The amount of the dihaloaromaticmonomer(s) per mole of the effective amount of the charged alkali metalsulfide may generally be from about 1.0 to about 2.0 moles, in someembodiments from about 1.05 to about 2.0 moles, and in some embodiments,from about 1.1 to about 1.7 moles. If desired, the dihaloaromaticmonomer may be charged into the second reactor with a relatively lowmolar ratio of the dihaloaromatic monomer to the alkali metal hydrogensulfide of the complex. For instance, the ratio of the dihaloaromaticmonomer to sulfur charged to the second reactor may be from about 0.8 toabout 1.5, and in some embodiments, from about 1.0 to about 1.2. Therelatively low ratio of the dihaloaromatic monomer to the alkali metalhydrogen sulfide of the complex may be favorable for the formation ofthe final high molecular weight polymer via the condensationpolymerization reaction. The ratio of solvent to sulfur in the secondstage may also be relatively low. For instance, the ratio of the alkalimetal hydrogen sulfide of the complex to the organic amide solvent inthe second stage (including the organic solvent added to the secondreactor and solvent remaining in the complex solution from the firstreactor) may be from about 2 to about 2.5. This relatively low ratio mayincrease the concentration of reactants in the second reactor, which mayincrease the relative polymerization rate and the per volume polymerproduction rate.

The second reactor may include a vapor outlet for removal of vaporduring the second stage in order to maintain the desired pressure level.For instance, the second reactor may include a pressure relief valve asis known in the art. Vapor removed from the second stage may becondensed and separated to recover unreacted monomer for return to thereactor. A portion of the water of the vapor may be removed to maintainthe near-anhydrous conditions of the second stage, and a portion of thewater may be returned to the second reactor. A small amount of water inthe second reactor may generate reflux in the top of the reactor, whichmay improve separation between the water phase and the organic solventphase in the reactor. This may in turn minimize loss of the organicsolvent in the vapor phase removed from the reactor as well as minimizeloss of hydrogen sulfide in the vapor stream due to absorption of thehydrogen sulfide by the highly alkaline organic solvent as discussedpreviously.

The second stage polymerization reaction may generally be carried out ata temperature of from about 200° C. to about 280° C., or from about 235°C. to about 260° C. The duration of the second stage may be, e.g., fromabout 0.5 to about 15 hours, or from about 1 to about 5 hours. Followingthe second stage polymerization reaction, the product solution thatexits second stage reactor may include the prepolymer, the organicsolvent, and one or more salts that are formed as a by-product of thepolymerization reaction. For example, the proportion by volume of theprepolymer solution exiting the second stage reactor of salt that isformed as a byproduct to the reaction may be from about 0.05 to about0.25, or from about 0.1 to about 0.2. Salts included in the reactionmixture may include those formed as a byproduct during the reaction aswell as other salts added to the reaction mixture, for instance as areaction promoter. The salts may be organic or inorganic, i.e., mayconsist of any combination of organic or inorganic cations with organicor inorganic anions. They may be at least partially insoluble in thereaction medium and have a density different from that of the liquidreaction mixture. According to one embodiment, at least a portion of thesalts in the prepolymer mixture that exits the second stage reactor maybe removed from the mixture. For instance, the salts may be removed byuse of screens or sieves as has been utilized in traditional separationprocesses. A salt/liquid extraction process may alternatively oradditionally be utilized in separating the salt from the prepolymersolution. In one embodiment, a hot filtration process may be utilized inwhich the solution may be filtered at a temperature at which theprepolymer is in solution and the salts are in the solid phase.According to one embodiment, a salt separation process may remove about95% or more of the salts including in the prepolymer solution that exitsthe second reactor. For instance, greater than about 99% of the saltsmay be removed from the prepolymer solution.

Following the prepolymer polymerization reaction in the second stage ofthe process and the filtration process, an optional third stage of theformation may take place during which the molecular weight of theprepolymer is increased in a third reactor. Input to the third reactormay include the prepolymer solution from the second reactor in additionto solvent, one or more dihaloaromatic monomers, and a sulfur-containingmonomer. For instance, the amount of the sulfur-containing monomer addedin third stage may be about 10% or less of the total amount required toform the product polyarylene sulfide. In the illustrated embodiment, thesulfur-containing monomer is sodium sulfide, but this is not arequirement of the third stage, and other sulfur containing monomers,such as an alkali metal hydrogen sulfide monomer may alternatively beutilized.

The third reaction conditions may be nearly anhydrous, with the ratio ofwater to the sulfur-containing monomer less than about 0.2, for instancebetween 0 and about 0.2. The low water content during the third stage ofthe process may increase the polymerization rate and the polymer yieldas well as reduce formation of undesired side reaction by-products asthe conditions are favorable for nucleophilic aromatic substitution, asdiscussed above. Moreover, as pressure increases in the third stage aregenerally due to water vaporization, low water content during this stagemay allow the third reaction to be carried out at a constant, relativelylow pressure, for instance less than about 1500 kPa. As such, the thirdreactor 104 need not be a high pressure reactor, which may providesubstantial cost savings to a formation process as well as decreasesafety risks inherent to high pressure reactors.

The reaction conditions within the third reactor may also include arelatively low molar ratio for the solvent to the sulfur-containingmonomer. For instance, the ratio of solvent to sulfur-containing monomermay be from about 2 to about 4, or from about 2.5 to about 3. Thereaction mixture of the third stage may be heated to a temperature offrom about 120° C. to about 280° C., or from about 200° C. to about 260°C. and the polymerization may continue until the melt viscosity of thethus formed polymer is raised to the desired final level. The durationof the second polymerization step may be, e.g., from about 0.5 to about20 hours, or from about 1 to about 10 hours. The weight averagemolecular weight of the formed polyarylene sulfide may vary as is known,but in one embodiment may be from about 1000 g/mol to about 500,000g/mol, from about 2,000 g/mol to about 300,000 g/mol, or from about3,000 g/mol to about 100,000 g/mol.

Following the third stage, and any desired post-formation processing,the polyarylene sulfide may be discharged from the third reactor,typically through an extrusion orifice fitted with a die of desiredconfiguration, cooled, and collected. Commonly, the polyarylene sulfidemay be discharged through a perforated die to form strands that aretaken up in a water bath, pelletized and dried. The polyarylene sulfidemay also be in the form of a strand, granule, or powder.

II. Purification System

Referring again to FIG. 1 , a purification system 600 is also employedfor washing the polyarylene sulfide slurry 501 formed via thepolymerization system 500. Within the system 600, the polyarylenesulfide slurry is contacted with a washing solution (not shown in FIG. 1), which results in a purified polyarylene sulfide stream 601 and aliquid washing product 603.

One or more solvents are typically incorporated into the washingsolution. Any of a variety of solvents may be employed, such as water,organic solvents, etc. In one embodiment, for example, water may beemployed, either alone or in combination with an organic solvent.Particularly suitable organic solvents include aprotic solvents, such ashalogen-containing solvents (e.g., methylene chloride, 1-chlorobutane,chlorobenzene, 1,1-dichloroethane, 1,2-dichloroethane, chloroform, and1,1,2,2-tetrachloroethane); ether solvents (e.g., diethyl ether,tetrahydrofuran, and 1,4-dioxane); ketone solvents (e.g., acetone andcyclohexanone); ester solvents (e.g., ethyl acetate); lactone solvents(e.g., butyrolactone); carbonate solvents (e.g., ethylene carbonate andpropylene carbonate); amine solvents (e.g., triethylamine and pyridine);nitrile solvents (e.g., acetonitrile and succinonitrile); amide solvents(e.g., N,N′-dimethylformamide, N,N′-dimethylacetamide, tetramethylureaand N-methylpyrrolidone); nitro-containing solvents (e.g., nitromethaneand nitrobenzene); sulfide solvents (e.g., dimethylsulfoxide andsulfolane); and so forth. Regardless of the particular solventsemployed, the entire solvent system (e.g., water andN-methylpyrrolidone) typically constitutes from about 80 wt. % to about99.9 wt. %, in some embodiments from about 85 wt. % to about 99.8 wt. %,and in some embodiments, from about 90 wt. % to about 99.5 wt. % of thewashing solution.

Other suitable materials may also be used in the washing solution, suchas stabilizers, surfactants, pH modifiers, etc. For example, basic pHmodifiers may be employed to help raise the pH of the washing solutionto the desired level, which is typically above 7, in some embodimentsfrom about 9.0 to about 13.5, and in some embodiments, from about 11.0to about 13.0. Suitable basic pH modifiers may include, for instance,ammonia, alkali metal hydroxides (e.g., sodium hydroxide, lithiumhydroxide, potassium hydroxide, etc.), alkaline earth metal hydroxides,etc., as well as combinations thereof. If desired, the solution may beheated prior to and/or during contact with the polyarylene sulfide toimprove washing efficiency. For example, the solution may be heated to atemperature of about 100° C. or more, in some embodiments about 110° C.or more, in some embodiments from about 120° C. to about 300° C., and insome embodiments, from about 130° C. to about 220° C. In certain cases,heating may be conducted at a temperature that is above the atmosphericpressure boiling point of a solvent in the mixture. NMP, for instance,has a boiling point at atmospheric pressure of about 203° C. In suchembodiments, the heating is typically conducted under a relatively highpressure, such as above 1 atm, in some embodiments above about 2 atm,and in some embodiments, from about 3 to about 10 atm.

The manner in which the polyarylene sulfide is contacted with thewashing solution may vary as desired. In one embodiment, for instance,the purification system 600 may include a vessel, such as a bath,sedimentation column, etc., within which the polyarylene sulfide iscontacted with the washing solution. Referring to FIG. 2 , FIG. 3 , FIG.4 , for instance, one embodiment of a sedimentation column 10 that canbe employed in the purification system 600 is shown in FIG. 2 that canbe configured to receive a polyarylene sulfide and washing solution. Thesedimentation column 10 may include an upper section 12 that includesliquid outlet 20, a middle section 14 that includes inlet 24, and alower section 16 that includes a solids outlet 22 and a liquid inlet 26.Though illustrated with a vertical arrangement, it should be understoodthat the sedimentation column may be utilized at other than a verticalarrangement, and the sedimentation column may be at an angle to verticalas long as solids flow through the sedimentation column from the inlet24 to the outlet 22 may be by gravitational force.

The upper section 12 and the lower section 16 may have cross sectionalareas that are larger than that of the middle section 14. In oneembodiment, the sections of the sedimentation column 10 may be circularin cross section, in which case the upper section 12 and lower section16 may have cross sectional diameters that are larger than the crosssectional diameter of the middle section 14. For instance, the uppersection 12 and the lower section 16 may have diameters that are fromabout 1.4 to about 3 times greater than the diameter of the middlesection. For instance, the upper and lower sections may independentlyhave diameters that are about 1.4, about 2, or about 2.5 times greaterthan the diameter of the middle section 14. The larger cross sectionalarea of the upper section 12 may prevent overflow of solids at theoutlet 20 and the larger cross sectional area of the lower section 16may prevent solids flow restriction at the outlet 22. It should beunderstood that the sedimentation column 10 is not limited to anyparticular geometric shape and the cross section of the sedimentationcolumn is not limited to circular. Moreover, the cross sectional shapeof each section of the sedimentation column may vary with regard to oneanother. For example, one or two of the upper section 12, the middlesection 14, and the lower section 16 may have an oval-shaped crosssection, while the other section(s) may be round in cross section.

The middle section 14 of the sedimentation column 10 may include aninlet 24 through which a polymer slurry may be fed to the sedimentationcolumn 10. The slurry may include the polyarylene sulfide in conjunctionwith other byproducts from the formation process, such as reactionsolvents (e.g., N-methylpyrrolidone), salt byproducts, unreactedmonomers or oligomers, etc. As shown in FIG. 3 , the inlet 24 meets awall 25 of the middle section 14 substantially tangent to the wall 25.The term “substantially tangent” as utilized herein may be determined bythe distance between the true tangent of the wall 25 of the middlesection 14 and the outer wall 27 of the inlet 24. When the inlet 24meets the wall 25 of the middle section at a perfect tangent, thisdistance will be 0. In general, this distance will be less than about 5centimeters, for instance less than about 3 centimeters. Placement ofthe inlet 24 such that the inlet 24 is substantially tangent to theouter wall 25 of the middle section 14 may prevent disruption of thefluid flow pattern within the sedimentation column 10. This may improvecontact and mass transfer between the downward flowing solids and theupward flowing liquid and may also prevent loss of solids through theoutlet 20 of the upper section 12. To further ensure that solids are notlost through the outlet 20, the inlet 24 may be placed in the middlesection 14 at a distance from the junction 23 where the middle section14 meets the upper section 12. For instance, the vertical distancebetween the midpoint of the inlet 24 and the junction 23 may be equal toor greater than about 5% of the total height of the middle section 14.For instance, the vertical distance between the midpoint of the inlet 24and the junction 23 may be from about 5% to about 50% of the totalheight of the middle section 14. The total height of the middle section14 is that distance between the junction 23, where the upper section 12meets the middle section 14 and the junction 21, where the middlesection 14 meets the lower section 16.

The inlet 24 may carry the slurry from a polymerization system 500 tothe middle section 14 of the sedimentation column 10. The middle section14 of the sedimentation column 10 may include an agitator 30 thatincorporates an axial shaft 31 and a series of stirring blades 32 alongthe axial length of the middle section 14. The agitator 30 may minimizechanneling of liquid within the sediment (fluidized bed) and maymaintain contact between the slurry contents and an upwardly flowingsolvent as well as maintain flow of the solids through the sedimentationcolumn 10. The stirring blades 32 may extend from the axial shaft 31toward the wall 25 of the middle section 14. In general, the stirringblades may extend at least half of the distance from the axial shaft tothe wall 25 and, in one embodiment, may extend almost all of the way tothe wall 25. In one embodiment, the sedimentation column may be free ofsedimentation plates or trays as have been utilized in previously knownsedimentation columns.

As shown, the axial shaft 31 may support a series of stirring blades 32along the length of the middle section 14. In general, at least twostirring blades 32 may extend from the axial shaft 31 in a balancedarrangement at each point of blade extension. This is not a requirement,however, and three, four, or more stirring blades may extend from theaxial shaft 31 at a single location along the shaft 31 or alternatively,a single blade may extend from a single location on the shaft 31 and thestirring blades may be offset from one another as they pass down thelength of the shaft 31 so as to maintain balance of the agitator 30during use. The axial shaft 31 may have stirring blades 32 extendingtherefrom at a plurality of locations along the shaft 31. For instance,the axial shaft may have stirring blades extending therefrom at fromabout 3 to about 50 locations along axial shaft 31, with two or morestirring blades 32 extending from the axial shaft at each location. Inone embodiment, the distribution of the blades along the axial shaft 31may be such that there are more blades in the fluidized bed section atthe bottom as compared to the number of blades in the upper portion ofsection 14. During operation, the axial shaft 31 may rotate at a speedthat is typically from about 0.1 rpm to about 1000 rpm, for instancefrom about 0.5 rpm to about 200 rpm or from about 1 rpm to about 50 rpm.

In the illustrated embodiment, a countercurrent flow is employed inwhich the polymer slurry flow is in a direction opposite to that of theflow of the washing solution. Referring again to FIG. 2 , for instance,the polymer slurry is fed to the middle section 14 of the sedimentationcolumn 10 via the inlet 24. The washing solution is, on the other hand,fed to the lower section 16 of the column 10 via an inlet 26. In thismanner, the washing solution can flow upwardly through the column as itcontacts the polymer slurry as it flows downwardly through the columntowards the solids outlet 22. If desired, the inlet 26 may include adistributor 35 that may enhance fluid flow through the solids andprevent solids from entering the inlet 26. The lower section 16 may alsohave a conical shape to concentrate the solids content at the outlet 22.The solids content of the slurry at the outlet 22 may generally be about20 wt. % or greater, or about 22 wt. % or greater in some embodiments.If desired, the washing solution may be heated prior to being fed to theinlet 26. In such embodiments, the sedimentation column may includeheating elements to maintain an elevated temperature during the washingprocess.

If desired, a fluidized bed may also be formed in the sedimentationcolumn with increasing concentration of solids from the top of the bedto the solids outlet 22. The fluidized bed height may be monitored andcontrolled so as to better control the residence time of solids in thesedimentation column. Through improved control of residence time for thesedimentation column, the efficiency of the separation process carriedout within the sedimentation column may be improved, which may translateto lower operational costs and improved separations. In addition,control of the fluidized bed height and residence time may help toprevent solids loss through the liquid outlet 20 of the upper section12. A sensor may be used to monitor the fluidized bed height within thesedimentation column 10. The sensor type is not limited and may be anysuitable sensor that may monitor the fluidized bed height including bothinternal sensors and external sensors. For example, sensors may utilize,without limitation, optical, infrared, radio frequency, displacement,radar, bubble, vibrational, acoustic, thermal, pressure, nuclear and/ormagnetic detection mechanisms to determine the fluidized bed heightwithin the sedimentation column 10. By way of example, in oneembodiment, an optical sensor 40 (e.g., a laser-based sensor including alaser source and a detector) may be located within the middle section 14of the sedimentation column 10, for instance near the level of the inlet24, which may detect reflection of the laser to determine relativedensity differences of the materials within the sedimentation column 10and thus convey information concerning the location of the top of thefluidized bed to a control system. The control system may relay thatinformation to a valve that may control flow of solids out of thesedimentation column at outlet 22 and/or a valve that may control flowof solids into the sedimentation column at inlet 24 so as to control thebed height. Surge tanks may also be included in the lines leading to andfrom the sedimentation column as is known to maintain control of thefluidized bed height. Other systems as are known in the art forcontrolling the height of a fluidized bed may alternatively be utilized,and the method and system utilized to control the bed height is notparticularly limited. The top of the fluidized bed may be at or near theinlet 24. To improve control of the residence time of the solids in thesedimentation column 10, the sediment bed height variation during theprocess may vary less than about 10% of the total height of the middlesection 14. For instance, the fluidized bed height variation during aprocess may be less than about 5% of the total height of the middlesection 14.

Though illustrated in FIG. 2 as a cylindrical column, the longitudinalcross-sectional shape of the middle section 14 is not limited to thisembodiment. For example, and as illustrated in FIG. 4 , middle sections14 a, 14 b, and 14 c of sedimentation columns may be straight or taperedwith increasing angles from a cylindrical middle section as illustratedat 14 a to increasing angles as shown at 14 b and 14 c. When tapered,the middle section may be wider at the top than at the bottom so as toincrease solids concentration at the bottom of the sedimentation columnwithout impeding transport of solids. That is, if the angle of taper istoo large, solids flow may be impeded at the wall of the middle section.A preferred taper angle will vary for each system depending upon flowrates, physical properties of the compounds to be carried within thesystem such as particle size and shape, as well as depending on columnmaterials and surface roughness.

In certain embodiments, the polyarylene sulfide can be contacted withthe washing solution in a single step. For example, a singlesedimentation column can be employed within which the polyarylenesulfide is washed. In alternative embodiments, however, multiple washingsteps may be employed. The washing solutions employed in the stages ofsuch a process need not be the same. In one embodiment, for instance,multiple sedimentation columns may be used in series, one or more ofwhich have a countercurrent flow such as described above.

Referring to FIG. 5 , for instance, one embodiment of a washing processis shown that employs three sedimentation columns 101 a, 101 b, and 101c in series. While the system may include one or more sedimentationcolumns having the design as described above, this is not a requirementof the disclosed systems. For instance, the system may includesedimentation columns that vary somewhat in design from thesedimentation column described above, e.g., the sedimentation columns ofthe system of FIG. 5 do not include upper and lower sections having alarger cross sectional area as compared to the middle section. In anycase, the system may include multiple sedimentation columns 101 a, 101b, and 101 c in series.

To initiate flow through the first sedimentation column 101 a, it may beinitially supplied with a polymer slurry via inlet 124 a. A firstwashing solution 302 may likewise be supplied to the column 101 a viainlet 126 a. The first washing solution 302 will generally flow upwardlythrough the column in a direction counter to that of the polymer slurryuntil reaching an outlet 120 a, where a first wash product 402 isremoved which may include the solution and dissolved compounds of thepolymer slurry feed. As shown, a solids outlet 122 a may thereafter feedsolids from the first sedimentation column 101 a to a slurry inlet 124 bof the second sedimentation column 101 b. A second washing solution 304will generally flow upwardly through the column 101 b via an inlet 126 bin a direction counter to that of the solids until reaching an outlet120 b, where a second wash product 404 is removed. Although notrequired, the second washing solution 304 can be a recycle stream thatis removed from the third sedimentation column 101 c via an outlet 120c. In the final sedimentation column 101 c, the solids from the secondsedimentation column may be fed via inlet 124 c. A third washingsolution 306 will generally flow upwardly through the column 101 c viaan inlet 126 c in a direction counter to that of the solids, which exitsthe column via an outlet 122 c to form the polymer product 406. As notedabove, the washing solutions employed in each stage of the washingprocess may vary, if desired. For instance, the first washing solution302 and/or second washing solution 304 may contain an organic solvent(e.g., N-methylpyrrolidone), either as the only solvent in the solutionor in combination with other types of solvents (e.g., water). The thirdwashing solution 306 may likewise contain water, either alone or incombination with other types of solvents.

Referring again to FIG. 1 , the washed polyarylene sulfide 601 may beremoved from the purification system 600 and dried according to anytechnique known in the art. Drying may occur at a temperature of fromabout 80° C. to about 250° C., in some embodiments from about 100° C. toabout 200° C., and in some embodiments, from about 120° C. to about 180°C. The purity of the resulting polyarylene sulfide may be relativelyhigh, such as about 95% or more, or about 98% or more. The purifiedpolyarylene sulfide may also have a relatively high molecular weight,such as weight average molecular weight of from about 30,000 to about60,000 Daltons, in some embodiments about 35,000 Daltons to about 55,000Daltons, and in some embodiments, from about 40,000 to about 50,000Daltons. The polydispersity index (weight average molecular weightdivided by the number average molecular weight) may likewise berelatively low, such as about 4.3 or less, in some embodiments about 4.1or less, and in some embodiments, from about 2.0 to about 4.0. Thenumber average molecular weight of the polyarylene sulfide may, forinstance, be from about 8,000 about 12,500 Daltons, in some embodimentsabout 9,000 Daltons to about 12,500 Daltons, and in some embodiments,from about 10,000 to about 12,000 Daltons. Molecular weight may bedetermined by converting the polymer to PPSO by oxidation with a mixtureof cold HNO₃ (50%) in a trifluoroacetic acid mixture, dissolving thePPSO in warm hexafluoroisopropanol (HFIP) for 1 hour, and then analyzingfor molecular weight by GPC equipped with PSS-hexafluoroisopropanol(HFIP) gel columns, which may be fitted with an HFIP-gel guard columnusing HFIP as mobile phase and refractive index as a detector.

The liquid washing product 603 may likewise be removed from thepurification system 600. As noted above, the washing product 603 may bederived from a single liquid stream, such as the liquid washing productexiting the outlet 20 in FIG. 2 . Likewise, the washing product 603 maybe derived from multiple liquid streams, such as the liquid washingproducts exiting the outlets 120 a, 120 b, and/or 120 c in FIG. 5 .Regardless, the washing product 603 generally contains an organicsolvent (e.g., N-methylpyrrolidone), which may be a residue of thepolymerization and/or washing processes. The washing product 603 mayalso contain water and/or various impurities, such as arylene sulfideoligomers, cyclic polyarylene sulfides, salts, etc.

III. First Distillation System

To help recover the organic solvent (e.g., N-methylpyrrolidone) from thewashing product 603, a first distillation system 700 is also employed inthe embodiment of FIG. 1 . The first distillation system 700 may includeone or a series of distillation columns to remove a substantial portionof the organic solvent and generate an organic solvent-rich stream 701.The organic solvent-rich stream 701 may, for instance, contain organicsolvents in an amount of about 70 wt. % or more, in some embodimentsabout 80 wt. % or more, and in some embodiments, from about 90 wt. % to100 wt. % of the stream. Examples of various suitable distillationtechniques are described, for instance, in U.S. Pat. No. 4,976,825 toIwasaki, et al. and U.S. Pat. No. 5,167,775 to Omori, et al.Distillation may, for instance, be performed at a temperature of fromabout 190° C. to about 300° C., and in some embodiments, from about 200°C. to about 290° C., and at a pressures of from about 50 to about 750Torr, and in some embodiments, from about 100 to about 500 Torr.

In addition to generating the organic solvent-rich stream 701, the firstdistillation system 700 also generates a waste sludge 703. The wastesludge 703 contains various impurities formed during the polymerizationprocess. Examples of such impurities include, for instance, volatileorganic compounds, sodium chloride, low molecular weight arylene sulfideoligomers, cyclic polyarylene sulfides, and fine polyarylene sulfideparticles having an average diameter of, for instance, less than 50micrometers. A “low molecular weight” oligomer typically refers to anarylene sulfide having a number average molecular weight of less thanabout 2,000 Daltons, in some embodiments about 1,500 Daltons or less,and in some embodiments, from about 100 to about 1,000 Daltons. Thepolydispersity index of such oligomers is typically high, such as aboveabout 7, in some embodiments about 9 or more, and in some embodiments,from about 10 to about 20. The weight average molecular weight maylikewise be about less than about 20,000 Daltons, in some embodimentsabout 15,000 Daltons or less, and in some embodiments, from about 1,000to about 12,000 Daltons. Cyclic polyarylene sulfides may also be presentin the waste slurry, which typically have the following general formula:

wherein,

n is from 4 to 30; and

R is independently hydrogen, alkyl, cycloalkyl, aryl, alkylaryl, or anarylalkyl radical having from about 6 to about 24 carbon atoms.

Furthermore, because the distillation system 700 is not capable ofremoving all of the organic solvent from the washing product 603, thewaste sludge 703 will still contain some portion of an organic solvent.As noted above, the organic solvent may be an amide solvent, such asN-methyl-2-pyrrolidone; N-ethyl-2-pyrrolidone; N,N-dimethylformamide;N,N-dimethylacetamide; N-methylcaprolactam; tetramethylurea;dimethylimidazolidinone; hexamethyl phosphoric acid triamide andmixtures thereof. Typically, the organic solvent constitutes from about10 wt. % to about 85 wt. %, in some embodiments from about 20 wt. % toabout 80 wt. %, and in some embodiments, from about 30 wt. % to about 50wt. % of the waste sludge 703. Likewise, the impurities typicallyconstitute from about 15 wt. % to about 90 wt. %, in some embodimentsfrom about 20 wt. % to about 80 wt. %, and in some embodiments, fromabout 50 wt. % to about 70 wt. % of the waste sludge 703.

IV. Organic Solvent Recovery

As illustrated in FIG. 1 , to recover the organic solvent, in someembodiments the waste sludge 703 can be acidified by combination with anacid, which can be supplied in an acid solution stream 706. The acidsolution stream 706 can include an acidifying agent, such as, withoutlimitation, sulfuric acid, phosphoric acid, hydrochloric acid, nitricacid, carboxylic acids, halogenated carboxylic acids, or combinationsthereof. In some embodiments, the acidifying agent can be a non-volatileagent, such as phosphoric acid, sulfuric acid, acetoacetic acid,beta-hydroxybutyric acid, etc., which can avoid acid contaminationissues in subsequent solvent recovery steps. The acid solution caninclude an acidifying agent in an amount of about 50 wt. % or more insome embodiments, for instance about 60 wt. % or more or about 70 wt. %or more, such as about 50 wt. % to about 90 wt. %, or about 70 wt. % toabout 85 wt. % in some embodiments.

When incorporated, the acid solution 706 can be combined with the wastesludge 703 in an amount such that upon combination, the acidified wastesludge 704 has a pH of less than 7, such as about 6 or less or about 5or less in some embodiments. For instance, the acidified waste sludge704 can include the acid solution in an amount of about 10 wt. % orless, such as about 7 wt. % or less, such as from about 3 wt. % to about5 wt. %, in some embodiments.

Acidification of the waste sludge can be carried out prior to orcontemporaneously with addition of the anti-solvent. Upon acidificationvarious components of the waste sludge can be affected. Exemplaryoligomer acidification schemes can include, without limitation:

Acidification of the waste sludge can reduce solubility of materialscontained in the waste sludge, such as oligomers, and thereby increasethe solid phase content of the sludge. Acidification of the waste sludgecan also increase agglomeration of particulates and fines, for instanceby reducing electrostatic repulsion between particles. Thus,acidification of the waste sludge can improve separation of solid fromliquids in subsequent separation steps and improve recovery of organicsolvents from the waste sludge.

The acidified waste sludge 704 (or the waste sludge 703 in thoseembodiments in which the waste sludge is not acidified) can be combinedwith an anti-solvent, which can be supplied in a separate stream 705.The source of the anti-solvent stream 705 may vary as desired. Incertain embodiments, for instance, the anti-solvent stream 705 may beused during purification of the polyarylene sulfide, such as a washingsolution employed in the purification system 600. Suitable anti-solventsmay include, for instance, lower alkanols having 1 to 10 carbon atoms(e.g., methanol, ethanol, n-propanol, isopropanol, n-butanol,isobutanol, t-butanol, etc.); ketones having 2 to 10 carbon atoms (e.g.,acetone); and alkanes having 5 to 10 carbon atoms (e.g., hexane), aswell as mixtures of any of the foregoing. Acetone is particularlysuitable when N-methylpyrrolidone is the organic solvent contained inthe waste sludge 703. Regardless, contact of the waste sludge 703 withthe anti-solvent 705, for instance in a stirred tank 710, forms adispersion 707, which includes a separate solid phase and liquid phase.The solid phase is formed by precipitation of a substantial portion, ifnot all, of the impurities in the waste sludge 703, such as the arylenesulfide oligomers, cyclic polyarylene sulfides, fine polyarylene sulfideparticles, and sodium chloride. In fact, in certain embodiments, suchcomponents may constitute about 70 wt. % or more, in some embodimentsabout 80 wt. % or more, and in some embodiments, from about 90 wt. % to100 wt. % of the precipitated solid phase. The liquid phase, on theother hand, generally contains the organic solvent employed duringformation of the polyarylene sulfide (e.g., N-methylpyrrolidone) and theanti-solvent. The anti-solvent typically constitutes from about 50 wt. %to about 90 wt. %, in some embodiments from about 60 wt. % to about 85wt. %, and in some embodiments, from about 65 wt. % to about 80 wt. % ofthe separated liquid phase, while the organic solvent typicallyconstitutes from about 10 wt. % to about 50 wt. %, in some embodimentsfrom about 15 wt. % to about 40 wt. %, and in some embodiments, fromabout 20 wt. % to about 35 wt. % of the separated liquid phase.Depending upon the specific liquid extractant used, the temperature ofthe mixture 1110 can be, for example, about 50° C. or greater, such asabout 60° C. or greater, about 70° C. or greater, or about 80° C. orgreater, such as from about 75° C. to about 100° C., or about 90° C. insome embodiment.

A solid/liquid separation system 800 can be used to readily separate thedispersion 707 into a solid phase 801 and a liquid phase 803. The system800 may employ any of a variety of known solid/liquid separationdevices, such as a filter, centrifuge, decanter, sedimentation column,etc. Suitable filtration devices may include, for instance, screenfilters, rotating filters, continuous rotary vacuum filters, continuousmoving bed filters, batch filters, etc. Although not shown in detailherein, the separated solid phase 801 can be removed as waste orrecycled and converted into a high molecular weight polyarylene sulfide.

In some embodiments, the solid/liquid separation system 800 can includemultiple separation steps. For instance, a filtration operation caninclude multiple washings of the solids with, e.g., an anti-solventand/or water, which can increase removal of organic solvent from afilter cake. For instance, a washing step can include washing of afilter cake with from about 100 grams to about 200 grams liquid perkilogram of solids, such as from about 150 grams to about 175 gramsliquid per kilogram sold. Optionally, multiple separation approaches canbe combined together such as multiple washings, centrifugations,filtrations, etc.

The liquid phase 803 can be subjected to a distillation process within asecond distillation system 900 to separate the organic solvent 903(e.g., N-methylpyrrolidone) from the anti-solvent 901 (e.g., acetone).The second distillation system 900 may employ one or a series ofdistillation columns. Distillation may, for instance, be performed at atemperature of from about 190° C. to about 300° C., and in someembodiments, from about 200° C. to about 290° C., and at a pressures offrom about 50 to about 750 Torr, and in some embodiments, from about 100to about 500 Torr. Regardless of the conditions employed, the organicsolvent 903 may be recovered and used in a variety of differentprocesses, if so desired. In certain embodiments, for example, theorganic solvent 903 may be recycled and used in the polymerizationsystem 500 during synthesis of the polyarylene sulfide. The organicsolvent 903 may also be used in the purification system 600 as a washingsolution.

The present invention may be better understood with reference to theExamples, set forth below.

EXAMPLE 1

Waste sludge from a polyarylene sulfide formation process was examined.The waste sludge included 17 wt. % solids (mostly NaCl), and 83 wt. %liquids.

The waste sludge was subjected to a filtration process following whichthe filter cake (53 wt. % solids) was subjected to a re-slurry operationto with either acetone (ATN) or water (H₂O) as the re-slurry liquid in aweight ratio of either 2:1 or 1:1.

Following re-slurry, the resulting sludge was examined at a shear rateof 4 s⁻¹ to compare viscosity characteristics. As shown in Table 1,below, slurries that included acetone as the re-slurry liquid had alower viscosity as compared to water-based re-slurries and at shearrates of 10 to 1,000 s⁻¹, the acetone re-slurries were conducive to bothpumping and mixing.

TABLE 1 Slurry liquid sludge:liquid Viscosity (cP) ATN 2:1 5699 ATN 1:1500 H₂O 2:1 6299 H₂O 1:1 2040

Multiple re-slurry/filter steps were carried out with the waste sludgeusing a 2:1 sludge:acetone re-slurry formation. FIG. 6 presents the %organic solvent (N-methylpyrrolidone, NMP) in the filtrate followingeach wash. As shown, there was less than 1% organic solvent present inthe filtrate from the third wash on.

The effect of pH on the filterability of the waste sludge was examined.An 85% solution of H₃PO₄ was utilized to adjust the waste sludge pH tovarying levels. FIG. 7 compares the filterability of the waste sludge atpH 7 and following acidification to pH 4. As shown, at pH 7, evenutilizing a higher pressure of 30 psi, the filterability of the sludgewas much lower than that of the waste sludge pH adjusted to pH 4 andfiltered at a pressure of 10 psi.

Ten samples of the waste sludge were pH adjusted to various levelsbetween pH 6.7 and pH 3.0. As shown in FIG. 8 , following acidification,the solids of the samples exhibited increased agglomeration of solids,particularly at a pH of about 5 and lower.

EXAMPLE 2

Waste sludge from a polyarylene sulfide formation process was examined.The waste sludge included 17 wt. % solids (mostly NaCl), and 83 wt. %liquids.

Samples of the waste sludge were combined with acetone and phosphoricacid in various amounts as shown in Table 2. The resulting mixture wasthen subjected to centrifugation at 3250 rpm. The time to form a filtercake in the centrifuge and the % NMP in the resulting filter cake areshown in Table 2.

TABLE 2 kg ATN/kg Time to filter % NMP in Sample # waste sludge % H₃PO₄(min)¹ cake² 1 1 0.7 1 0.7 2 0.5 0.7 2 1.3 3 0.25 0.7 4 2.5 4 0 0.7 103.4³ 5 0 0.3 15 6.9³ 6 0 0.2 16 4.2 ¹at 3250 rpm bench centrifuge filter²after 2 washes with acetone (2.5 = total ATN used for wash equal to 20%of feed ³reduced to <1% NMP if 5.2 wash

As can be seen, the time for filtration improved with acidification,even without initial acetone dilution, and the residual organic acid inthe filter cake was substantially decreased upon both acidification andanti-solvent treatment.

While particular embodiments of the present disclosure have beenillustrated and described, it would be obvious to those skilled in theart that various other changes and modifications may be made withoutdeparting from the spirit and scope of the disclosure. It is thereforeintended to cover in the appended claims all such changes andmodifications that are within the scope of this disclosure.

What is claimed is:
 1. A method for recovering an organic solvent from awaste sludge developed in formation of a polyarylene sulfide, the methodcomprising: contacting the waste sludge with an anti-solvent to form adispersion that includes a solid phase and a liquid phase, the wastesludge containing from about 10 wt. % to about 70 wt. % of an organicsolvent; separating the solid phase from the liquid phase, wherein theliquid phase contains the organic solvent and the anti-solvent; andsubjecting the liquid phase to a first distillation process during whichthe organic solvent is separated from the anti-solvent and recovered. 2.The method of claim 1, further comprising generating the waste sludgefrom a second distillation process.
 3. The method of claim 2, wherein aninput to the second distillation process includes a liquid washingproduct obtained upon washing a polyarylene sulfide slurry formed in apolyarylene sulfide polymerization process.
 4. The method of claim 3,wherein the polyarylene sulfide slurry is washed with a washing solutionthat contains the organic solvent.
 5. The method of claim 3, wherein thepolyarylene sulfide slurry is contacted with a washing solution within asedimentation column.
 6. The method of claim 3, wherein the anti-solventis derived from a stream used during washing of the polyarylene sulfideslurry.
 7. The method of claim 1, further comprising acidifying thewaste sludge, wherein the acidified waste sludge has a pH of less thanabout
 7. 8. The method of claim 7, wherein the waste sludge is acidifiedprior to contacting the waste sludge with the anti-solvent.
 9. Themethod of claim 1, wherein the organic solvent is an organic amidesolvent.
 10. The method of claim 9, wherein the organic amide solvent isN-methylpyrrolidone.
 11. The method of claim 1, wherein the waste sludgecontains an arylene sulfide oligomer, a cyclic polyarylene sulfide,sodium chloride, fine polyarylene sulfide particles, volatile organiccompounds, or a combination thereof.
 12. The method of claim 1 whereinthe anti-solvent includes a lower alkanol having 1 to 10 carbon atoms, aketone having 2 to 10 carbon atoms, an alkane having 5 to 10 carbonatoms, or a combination thereof.
 13. The method of claim 1, wherein theanti-solvent includes acetone.
 14. The method of claim 1, wherein theanti-solvent constitutes from about 50 wt. % to about 90 wt. % of theseparated liquid phase and the organic solvent constitutes from about 10wt. % to about 50 wt. % of the separated liquid phase.
 15. The method ofclaim 1, further comprising recycling the recovered organic solvent intoa process for forming the polyarylene sulfide, a process for washing thepolyarylene sulfide, or a combination thereof.
 16. The method of claim1, wherein the polyarylene sulfide is a polyphenylene sulfide.
 17. Asystem comprising: an anti-solvent stream that is configured to contacta waste sludge developed in formation of a polyarylene sulfide to form adispersion that includes a solid phase and a liquid phase; asolid/liquid separation system that is configured to separate thedispersion into a solid phase and a liquid phase containing an organicsolvent and the anti-solvent; and a first distillation system that isconfigured to recover the organic solvent from the separated liquidphase.
 18. The system of claim 17, further comprising: a polymerizationsystem that is configured to generate the polyarylene sulfide; apurification system that is configured to receive a polyarylene sulfideslurry containing the polyarylene sulfide and generate a liquid washingproduct; and a second distillation system that is configured to receivethe liquid washing product and generate the waste sludge.
 19. The systemof claim 17, further comprising an acid solution stream containing anacidifying agent, the acid solution stream containing an acidifyingagent, the acid solution stream being configured to contact the wastesludge to form an acidified waste sludge.
 20. The system of claim 19,wherein the acid solution stream is configured to contact the wastesludge prior to the anti-solvent stream contacting the waste sludge.